THE PRESENT INVENTION relates to an impact detector and more particularly relates to an impact detector for use in detecting an impact on the side of a motor vehicle.
It has been proposed previously to provide impact detectors in motor vehicles to detect when a motor vehicle suffers from an impact. Such an impact detector may be used to activate a safety device within the motor vehicle such as a so-called "air-bag" or a safety-belt pretensioner.
Many of the prior proposed impact detectors detect any impact on the vehicle, but it has now been found that there is a need for impact detectors which specifically detect an impact on the side of a vehicle. Such impact detectors may be utilised to trigger specially located "air-bags" which can protect the driver of a vehicle, or a passenger in a vehicle, especially from a side impact.
German Specification DE-OS-3716168 discloses arrangements for detecting a side impact. In the arrangements disclosed in this Specification two elongate elements extend transversely across the door of a motor vehicle adjacent the outer skin of the door. In one embodiment the transversely extending elements are rigid and are mounted on Piezo electric transducers so that when the outer skin of the door is deformed, and touches the transverse members, the transducers generate appropriate signals. In an alternative embodiment the elongate transverse members comprise light guides which direct light from a photo diode to photo transistors, and again the arrangement is such that when the outer skin of the vehicle door engages the members, light is no longer transmitted through the members and thus appropriate signals can be generated. In each of these embodiments the speed of the deformation of the outer skin of the door is measured by determining the time lapse between two specific degrees of deformation, that is to say the degree of deformation when the outer skin engages the first transverse member and the subsequent degree of deformation when the skin of the door engages the second transverse member.
GB-A-2225660 discloses an alternative arrangement in which a pressure switch is actuated in response to deformation of the outer skin of the door.
It is to be noted that in the prior art arrangements discussed above it is the deformation of the outer skin of the door that is detected. It has now been found that such devices are not totally satisfactory, in that the devices respond to any deformation of the outer skin of the door. It is to be noted that conventionally the outer skin of the door of a motor vehicle is formed of a pressed steel sheet which is made to be as thin as possible. Thus this sheet will be deformed significantly by an impact which only possesses relatively low energy. For instance, the outer skin of the door of a motor vehicle can be significantly deformed by a high speed blow from a light item, such as a kick from a foot. Also, the outer skin of the door of a car can be significantly deformed if the door is brought into contact with a fixed object when the door is opened, as may happen when the car is parked immediately adjacent a lamp post or the like. The reason for this is that the outer skin of the door of the vehicle does not have any significant inherent strength and the degree of deformation in an impact is dependent upon the speed of impact and is almost independent of the energy of the impact. The present invention therefore seeks to provide an arrangement in which a passive sensor element is utilised which can make it possible to distinguish between a low energy impact, which would not be sufficient to warrant inflation of the special air-bag and a high energy impact which would make it appropriate to inflate the air-bag.